1. Field of the Invention
The present invention relates to image transmission systems, and to methods of coding and decoding video signals for such systems.
2. Description of Related Art
Image transmission systems for video conferencing and video telephony are required to convey image information at low bit rates, typically at 64 kbit/sec. The image information transmitted over such systems must enable acceptable resolution of image detail and frequency of up-dating at a receiver, and for this purpose images to be transmitted may be presented for encoding as a succession of image frames, each made up of 288 lines of 360 pixels or pels, at a rate of ten frames per second. If the brightness and colour of each of these pixels is to be rendered at all accurately, full encoding of such a succession of image frames would give rise to a data rate of 10 Mbit/sec or more, and clearly only a very small proportion of this data could be transmitted over a 64 Kbit/sec channel.
Reduction of the data rate may be achieved by exploiting spatial redundancy within individual frames and temporal redundancy between successive frames. One way of taking advantage of temporal redundancy is to decompose each current frame into blocks of 8.times.8 pixels and, in respect of each such block, to look for a similar block in a portion of the succeeding frame in a "search window" of, say, .+-.7 pixels centred on the block under examination, in order to identify any movement of image elements between frames. A motion vector mapping each block to the similar block in the succeeding frame can then be associated with each block. Following this scheme it is possible to reconstruct the succeeding frame by translating blocks from the current frame according to the associated motion vectors. This technique is described, for example, by S. Kappagantula and K. R. Rao in IEEE Transactions on Communications, Volume Comm-33, No. 9, September 1985, pages 1011 to 1015.